# React Quick Reference

*Components, hooks, state, effects, patterns*

> Source: React Documentation (react.dev) · MIT

## JSX Basics

### Expressions & Attributes

```
const name = "Alice";
const el = <h1>Hello, {name}!</h1>;
const img = <img src={url} alt="photo" />;
```

### JSX Rules

| Command | Description |
|---------|-------------|
| `{expression}` | Embed any JS expression |
| `className` | Use instead of `class` |
| `htmlFor` | Use instead of `for` |
| `style={{color: 'red'}}` | Inline styles as object |
| `<Component />` | Self-closing tags required |
| `<> ... </>` | Fragment (no extra DOM node) |

## Components

### Function Components

```
function Greeting({ name }) {
  return <h1>Hello, {name}!</h1>;
}

// Arrow function variant
const Greeting = ({ name }) => (
  <h1>Hello, {name}!</h1>
);
```

### Props

```
function Card({ title, children }) {
  return (
    <div className="card">
      <h2>{title}</h2>
      {children}
    </div>
  );
}

<Card title="Welcome">
  <p>Content here</p>
</Card>
```

### Default Props

```
function Button({ label = "Click me", onClick }) {
  return <button onClick={onClick}>{label}</button>;
}
```

## State (useState)

### Basic State

```
import { useState } from "react";

function Counter() {
  const [count, setCount] = useState(0);
  return (
    <button onClick={() => setCount(count + 1)}>
      Count: {count}
    </button>
  );
}
```

### Functional Updates

```
setCount(prev => prev + 1); // use prev state
setItems(prev => [...prev, newItem]); // arrays
setUser(prev => ({...prev, name: "Bob"})); // objects
```

### State Rules

| Command | Description |
|---------|-------------|
| `Immutable updates` | Never mutate state directly |
| `Async batching` | Updates may be batched |
| `Functional form` | Use `prev =>` when depending on prior state |

## Effects (useEffect)

### Effect Patterns

```
import { useEffect } from "react";

// Run on every render
useEffect(() => { /* ... */ });

// Run once on mount
useEffect(() => { /* ... */ }, []);

// Run when deps change
useEffect(() => { /* ... */ }, [count]);

// Cleanup on unmount
useEffect(() => {
  const id = setInterval(tick, 1000);
  return () => clearInterval(id);
}, []);
```

## Lists & Keys

```
function TodoList({ items }) {
  return (
    <ul>
      {items.map(item => (
        <li key={item.id}>{item.text}</li>
      ))}
    </ul>
  );
}
```

*Keys must be stable, unique IDs -- avoid array index as key*

## Event Handling

### Events

```
<button onClick={handleClick}>Click</button>
<button onClick={() => handleDelete(id)}>Del</button>
<input onChange={(e) => setVal(e.target.value)} />
<form onSubmit={(e) => {
  e.preventDefault();
  handleSubmit();
}}>
```

### Common Events

| Command | Description |
|---------|-------------|
| `onClick` | Mouse click |
| `onChange` | Input value change |
| `onSubmit` | Form submission |
| `onKeyDown` | Key press |
| `onFocus / onBlur` | Focus gained / lost |
| `onMouseEnter` | Mouse enters element |

## Conditional Rendering

```
// Ternary
{isLoggedIn ? <Dashboard /> : <Login />}

// Logical AND (short-circuit)
{hasError && <ErrorBanner />}

// Early return
function Page({ user }) {
  if (!user) return <Login />;
  return <Dashboard user={user} />;
}
```

## Hooks

### useRef

```
const inputRef = useRef(null);
// Access: inputRef.current.focus();
<input ref={inputRef} />
```

*useRef persists values across renders without triggering re-render*

### useMemo & useCallback

```
// Memoize expensive computation
const sorted = useMemo(
  () => items.sort(compareFn),
  [items]
);

// Memoize callback
const handleClick = useCallback(
  () => setCount(c => c + 1),
  []
);
```

### useContext

```
import { useContext } from "react";
const value = useContext(MyContext);
```

## Custom Hooks

```
function useLocalStorage(key, initial) {
  const [value, setValue] = useState(() => {
    const saved = localStorage.getItem(key);
    return saved ? JSON.parse(saved) : initial;
  });

  useEffect(() => {
    localStorage.setItem(key, JSON.stringify(value));
  }, [key, value]);

  return [value, setValue];
}

// Usage
const [name, setName] = useLocalStorage("name", "");
```

*Custom hooks must start with 'use'*

## Context API

### Create & Provide

```
import { createContext, useContext } from "react";

const ThemeCtx = createContext("light");

function App() {
  return (
    <ThemeCtx.Provider value="dark">
      <Page />
    </ThemeCtx.Provider>
  );
}
```

### Consume

```
function Page() {
  const theme = useContext(ThemeCtx); // "dark"
  return <div className={theme}>...</div>;
}
```